FWIW, I've shortened the buttstock material to a length desired, allowing for the thickness of whatever BP ot recoil pad was to be installed later, with an ordinary saw, after ensuring that straight marks, etc, were scribed onto the proposed cutting line.

I then fit a 1-1/2" thick piece of wood (pine, usually), carved to fit closely inside the open/hollow end & flush with the new, cutt-off end of the buttstock.

Although I epoxy the wood piece in, before I do, I fill the hollow stock space with some construction spray insulating foam (for around house windows, etc) - then also secure the wood with a sling swivel woodscrew-type QD stud up through the outside of the stock's bottom line into the wood.

The BP or pad can then be fitted & screwed/glued to the end of the wooden plug.

BTW - Some synthetic stocks have molded-in bosses, inside the top & bottom edges, so that the issue BP or pad can be screwed on, and if they're not entirely removed via the shortening process, either they will need to be ground out from the inside (dremel tool) or the wood plug carved to slip by them.